In producing hydrocarbons or the like from subterranean formations, it is now common to use boreholes or laterals which extend horizontally into the producing formation. Such horizontal boreholes are often completed by "casing" the borehole with a liner having pre-formed openings therein (e.g. perforations, slots, etc.). This uncemented liner serves (a) to stabilize the formation and prevent caving of the borehole during production and (b) to provide access into the borehole for a variety of downhole, work-over tools which may be needed during the production life of the well.
Frequently, in completions of this type, it is necessary to "clean" the openings in the liner and/or the formation behind the liner to improve the production of fluids from the formation. This cleaning operation is routinely carried out by injecting a "cleaning fluid", e.g. water, gas, etc., through the openings in the liner and into the formation behind the liner. As will be recognized in the art, a wide variety of downhole tools, sometimes called "wash tools" have been proposed for carrying out this type of operation.
For example, in one group of wash tools, a nozzle or the like is attached to the bottom of a workstring on which the nozzle is lowered down the well and into the liner. Fluid, e.g. water, gas, etc. is then flowed down the workstring and out the nozzle to thereby "wash" the openings in the liner and/or flush the accumulated material in the formation behind the liner away from the openings. The nozzle may be either fixed or may rotate as the wash fluid passes therethrough e.g. see U.S. Pat. Nos. 2,120,988; 2,186,309; 4,037,661; 4,909,325; and 5,533,571.
To provide sufficient clearance for tools of this type to be run into and out of the well, the outer diameter of the tool is normally substantially less than the smallest inside diameter expected to be encountered within the well; e.g. landing nipples, etc. This requirement can result in a relatively large annulus between the tool and the inner wall of the liner when the tool is in an operable position within the liner. Accordingly, when fluid flows out the nozzle during a cleaning operation, a substantial portion of that fluid will not achieve its objective, i.e. clean the openings and/or formation, but instead will by-pass the tool and flow upward through this large annulus back to the surface. This loss of cleaning fluid obviously adversely affects the efficiency of over-all operation.
To reduce the amount of returned fluid during a cleaning operation, other wash tools have been proposed which include some type of sealing means designed to block upward fluid flow between the tool and the liner. In those tools where the seals are expandable to block flow, e.g. see U.S. Pat. No. 2,290,141, the tool is complicated and requires a certain amount of sophistication for its operation. In tools having fixed seals thereon, e.g. see U.S. Pat. Nos. 1,379,815 and 4,763,728, their outer diameters are such that there is little clearance, if any, between the tool and the borehole which makes it difficult to run such tools into and out of a borehole, especially a horizontal borehole.